A metal base circuit board comprising an insulating layer made of e.g. an epoxy resin containing an inorganic filler provided on a metal plate, and having a circuit provided thereon, is excellent in heat dissipation properties and is thereby used as a circuit board on which a highly heat generating electronic component is to be mounted.
FIG. 1-1 illustrates one example of a conventional hybrid integrated circuit comprising a metal base circuit board and a semiconductor device mounted thereon. In its structure, a power semiconductor 2 and a signal conditioning semiconductor (hereinafter sometimes referred to as a control semiconductor) 1 are mounted on circuits 3 made of e.g. copper via solder. Further, in its structure, the semiconductors are electrically connected via the copper circuit or via a bonding wire 5. When the power conductor 2 is mounted on the circuit 3, a heat sink 6 is interposed therebetween in many cases so as to accelerate heat dissipation and to prevent an increase in temperature of the power semiconductor which may lead to malfunction (Japanese Patent No. 3156798).
In the hybrid integrated circuit having the above structure, each of the semiconductors and a metal base board are electrically and electrostatically connected. Accordingly, when it is intended to increase the operating frequency (several hundred kHz or above), signals (digital signals) from the control semiconductor 1 are distorted, the power semiconductor 2 malfunctions, and the apparatus itself does not operate as designed in some cases. As a countermeasure against the above, lift-off may be employed so as to eliminate bad influences of the capacitance resulting from a metal board, but there are many problems in view of cost and assembling.
Further, FIG. 1-2 illustrates an example of another known hybrid integrated circuit. There has been an attempt to reduce bad influences of the capacitance resulting from a metal board by increasing the thickness of an insulating layer on which a control semiconductor is mounted as compared with the structure shown in FIG. 1-1. However, in preparation of such a hybrid integrated circuit or a metal base circuit board to be used therefor, it is required to prepare insulating layer portions with different thicknesses partially, which makes the process complicated, and which enlarges the obtained hybrid integrated circuit in an upward direction. Further, there has been an attempt to achieve the above structure by laminating a resin substrate or the like having a circuit preliminarily provided thereon on a metal plate or on a metal plate having an insulating layer provided thereon (also called a metal base board) instead of the increase in thickness of the insulating layer (Japanese Patent No. 2608980), however, the above problem still remained unsolved.
With respect to a circuit board on which a hybrid integrated circuit is mounted, along with high integration of a semiconductor device, a means becomes in the mainstream that a power semiconductor device, etc. is miniaturized and further, a resistor chip, etc. in addition to various types of semiconductor devices, is mounted on the same board. As board characteristics required are different among the respective semiconductor devices and electronic components, a circuit board having characteristics partially corresponding to the respective semiconductor devices and electronic components has been required.
Accordingly, a circuit board comprising different types of insulating layers combined has been proposed, as disclosed in JP-A-6-90071, for example. However, it costs a lot to produce such a composite insulating board since a process of its production is complicated, and further, it is not technically easy to complicatedly combine different types of insulating layers in a small area, whereby considerable miniaturization of a circuit board has been hardly achieved.
Further, in a hybrid integrated circuit employing a metal base circuit board, usually, both power semiconductor and control semiconductor to control operation of the power semiconductor are mounted on circuits of the metal base circuit board. As characteristics required for the metal base circuit board, the former is required to be excellent in thermal conductivity even if it has a slightly high capacitance, and the latter is required to have a very low capacitance even at the sacrifice of the thermal conductivity.
However, it was found that if a control semiconductor is mounted on a circuit which is designed to have a very low capacitance, concurrent heat dissipation from the control semiconductor is insufficient, and no sufficient operating time can be secured in some cases.